How do I execute async.map function in order of array contents

Question

I've got the following code routine that works great. The only problem is that I need the results to come back in the same order of the links array. For example I need the results of the google.com link to come back first, then yahoo, etc. The code currently returns in a "random" order.

var Nightmare = require('nightmare');
var async = require('async');
var links = [
    "http://www.google.com",
    "http://www.yahoo.com",
    "http://www.bing.com",
    "http://www.aol.com",
    "http://duckduckgo.com",
    "http://www.ask.com"
  ];

var scrape = function(url, callback) {
  var nightmare = new Nightmare();
  nightmare.goto(url);
  nightmare.wait('body');
  nightmare.evaluate(function () {
    return document.querySelector('body').innerText;
  })
  .then(function (result) {
    console.log(url, result);
  })
  nightmare.end(function() {
    callback();
  });
}

async.map(links, scrape);

UPDATE: Thanks @christophetd. Here is my revised working code:

var Nightmare = require('nightmare');
var async = require('async');
var links = [
    "http://www.google.com",
    "http://www.yahoo.com",
    "http://www.bing.com",
    "http://www.aol.com",
    "http://duckduckgo.com",
    "http://www.ask.com"
  ];

var scrape = function(url, callback) {
  var nightmare = new Nightmare();
  nightmare.goto(url);
  nightmare.wait('body');
  nightmare.evaluate(function () {
    return document.querySelector('body').innerText;
  })
  .then(function (result) {
    callback(null, url+result);
  });
  nightmare.end();
}

async.map(links, scrape, function (err, results) {
  if (err) return console.log(err);
  console.log(results);
})

what you describe is not async... can you re-order them once you have them all? — dandavis
– dandavis, Commented May 24, 2016 at 20:58

christophetd · Accepted Answer · 2016-05-24 21:11:58Z

5

From the official async documentation :

the results array will be in the same order as the original collection

Which is pretty easy to verify:

// This function waits for 'number' seconds, then calls cb(null, number)
var f = function (number, cb) {
    setTimeout(function () {
        cb(null, number)
    }, number * 1000)
}

async.map([4, 3, 2, 1], f, function (err, results) {
    console.log(results); // [4, 3, 2, 1]
})

As you can see in the code above, even if the processing of the argument 4 by f takes more time than the element 3, it will still be first in the results.

In the case of your code, writing:

async.map(links, scrape, function (err, results) {
    if (err) {
        // handle error, don't forget to return
    }
    // results will be in the same order as 'links'
})

Should give you the expected result.

answered May 24, 2016 at 21:11

christophetd

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1 Comment

Matt Over a year ago

Thanks, this was perfect. I had seen the answer you referenced in the docs, but for some reason it didn't make sense until I saw your answer.

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